The present invention relates in general to the field of electrochemical sensors for detecting and quantifying an analyte in the presence of an interferent. In particular, the invention relates to electrochemical toxic gas sensors which are used in an environment where there may be an interferent gas which acts to shift the potential of the electrochemical sensor""s reference electrode. A specific example would be a carbon monoxide sensor for use in environments where there may also be hydrogen present.
Electrochemical gas sensors are well known for detecting and quantifying toxic gases such as carbon monoxide, hydrogen sulphide, nitrogen oxides, chlorine, sulphur dioxide and the like. Such sensors are electrochemical cells; more specifically, they operate in an amperometric mode providing a current output which is related to the concentration of the particular analyte gas. A known sensor is shown in schematic form in FIG. 1. The sensor shown generally as 1 comprises a working electrode 2 which is typically made by sintering at an elevated temperature a mixture of catalyst (e.g. Platinum Black) and a suspension of PTFE, then pressing the sintered mixture onto a microporous PTFE membrane 3.
A disc shaped reference electrode 4 is most usually constructed similarly, again being typically a Platinum Black electrode on a microporous PTFE membrane. The counter electrode 5 is again a Platinum Black electrode in contact with an electrolyte reservoir 6, typically sulphuric acid with a concentration in the range of three to seven Molar. The electrodes are connected with electronic circuitry outside the sensor by suitable electrical contacts. A potentiostatic circuit 7 is provided which can maintain the potential of the working electrode at a constant value with respect to the reference electrode. Cell electrolyte provides ionic contact between the electrodes. A diffusion barrier 8 controls entry of gas into the sensor through a hole 9 into gas space 10.
It is known that the variation of the working electrode current (I) with applied potential (E) is broadly as shown schematically in FIG. 2. FIG. 2 illustrates that there is a plateau region of several hundred millivolts where there is very little dependence of I on E. This region of the current voltage curve is known as the diffusion, or transport limited, current plateau and occurs because the current is controlled by the diffusional flux or mass transport of the electro-active species. In this case, the toxic gas diffuses to the working electrode and this flux is independent of the potential applied to the electrode. Before and after the plateau region the current is controlled by the electrochemical kinetics of the electrode/electrolyte interface and in these regions the current is dependent on electrode potential. Therefore, toxic gas sensors are operated in the diffusion limited plateau region. For many toxic gas sensors it is found that a zero voltage applied to the working electrode with respect to the reference electrode conveniently corresponds to part of the plateau region. The transport limited current has been shown theoretically to be linearly dependent on the concentration of the toxic gas in the external environment and so an electrochemical cell operating in this mode can be effectively used for monitoring toxic gas levels.
The electrochemical reactions occurring in the cell can be illustrated by referring to what happens in a carbon monoxide sensor. At the working electrode the CO is oxidised:
CO+H2Oxe2x86x92CO2+2H++2exe2x88x92xe2x80x83xe2x80x83(R1)
At the counter electrode there is a counterbalancing reduction which can be represented as the reduction of hydrogen:
2H++2exe2x88x92xe2x86x92H2xe2x80x83xe2x80x83(R2)
Thus the overall cell reaction is the sum of (R1) and (R2):
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(R3)
However, it is well known that in this class of carbon monoxide sensors, the predominating counter electrode reaction in air is the reduction of oxygen.
Although these sensors operate well over a large temperature range, there is a serious drawback to this type and geometry of sensor. Hydrogen gas cohabits frequently where carbon monoxide is measured and the type of carbon monoxide gas sensor illustrated in FIG. 1 will show a hydrogen cross-sensitivity of 30-60% (ie 100 ppm of hydrogen will create a current equivalent to 30-60 ppm of CO). Here, hydrogen acts as an interferent.
A strategy used for dealing with chemical interferents in other classes of chemical sensor is scrubbing the interferent with a chemical filter. However, this cannot be readily achieved for hydrogen and so the prior art has proposed several alternative ways of alleviating this source of error.
Firstly, it is known to provide a second working electrode which responds only to hydrogen. The first working electrode is sensitive to both hydrogen and carbon monoxide and the difference between the two currents, properly scaled and calibrated, should be the corrected carbon monoxide concentration. However, calibration is difficult and the sensor is prone to drift due to non-equivalent changes of catalytic activity in the two working electrodes, reducing accuracy and reliability.
Secondly, a catalyst with reduced activity appears to affect the hydrogen sensitivity more than the carbon monoxide sensitivity and so sensors with reduced catalytic activity display reduced hydrogen cross-sensitivity However, this of course reduces the signal strength and the resultant sensor shows poor performance at sub-ambient temperatures; for example a CO sensor with poor activity will show a hydrogen sensitivity of only 10%(relative to CO) but with 40% CO sensitivity at xe2x88x9220 C. (compared to the sensitivity at 20 C.) while a more active catalyst will show a 25 to 40% hydrogen sensitivity but with 80% CO sensitivity at xe2x88x9220 C. (compared to the sensitivity at 20 C.).
Thirdly, Endress and Hauser developed a low hydrogen cross sensitive carbon monoxide sensor which uses an additive in the electrolyte. However, in time the irreversibility of the reaction with the additive leads to increased cross-sensitivity reference required.
Fourthly, a bias voltage can be imposed to offset the sensor into a regime on the I-E curve where the hydrogen oxidation is less favoured than the CO oxidation, since their I-E curves will be different. However this bias voltage must be applied at all times which is a problem with portable instruments with sometimes months between use: the back-up battery for continuous biasing may be fully discharged and no longer supplying a bias voltage. This correction has been used in commercially available breath analysers for clinical CO detection.
Therefore, each of the presently available solutions results in difficult calibration, long-term drift of signal, poor temperature performance or battery problems.
The present invention aims to provide an electrochemical cell for detection of an analyte in the presence of an interferent in which the effect of the interferent is reduced or obviated. The invention aims to achieve this goal whilst providing a sensor which remains practical to calibrate, does not suffer from working electrode long-term sensitivity drift, poor temperature performance or other undesirable characteristics.
Within this specification the term xe2x80x9canalytexe2x80x9d refers to a particular chemical species which is to be measured and the term xe2x80x9cinterferentxe2x80x9d refers to a second distinct chemical species which would undergo an electrochemical reaction producing an electrical signal which leads to a signal which resembles that due to the analyte.
According to a first aspect of the present invention there is provided an electrochemical cell for sensing an analyte in the presence of an interferent, the electrochemical cell comprising a reference electrode and a working electrode connected by a potentiostatic circuit, the analyte reacting at the working electrode giving a first component of current, the interferent undergoing an electrochemical reaction giving a second component of current, the presence at the reference electrode of the interferent leading to a shift in the potential of the reference electrode, wherein the shift in the potential of the reference electrode and the action of the potentiostatic circuit leads to a third component of current, wherein properties of the electrochemical cell which affect the magnitude of the third component of current are selected so that the third component of current acts to partially, completely or over compensate for the second component of current.
Typically, the interferent might cohabit with the analyte gas or be generated by the counter electrode.
Preferably, the properties of the electrochemical cell which affect the magnitude of the third component of current are one or more of the capacitance of the working electrode, the metal on the reference electrode, the oxidation state of the metal on the reference electrode, the redox couple, capacitance or geometry of the reference electrode.
Preferably, the electrochemical cell is a gas sensor.
Preferably also, the analyte is carbon monoxide and the interferent is hydrogen.
More preferably, working electrode is a circular gas porous disk.
Preferably also, the reference electrode is an annulus, coaxial with the working electrode. More preferably, the reference electrode is at least partially exposed to the analyte gas without electrolyte between the electrode and analyte gas.
Preferably, the capacitance of the working electrode is at least 1 mF.
Typically, the electrochemical cell is used periodically. Usually this is for less than 15 minutes.
According to a second aspect of the present invention there is provided an electrochemical cell for sensing an analyte in the presence of an interferent, the electrochemical cell comprising a working electrode and a first reference electrode connected by a potentiostatic circuit, the interferent not affecting the reference electrode thus avoiding a shift in the potential of the reference electrode, the analyte reacting at the working electrode giving a first component of current, the interferent undergoing an electrochemical reaction giving a second component of current characterized in that there is further provided a second reference electrode, configured so as to be exposed to the interferent, the interferent causing a shift in the potential of the second reference electrode, the second reference electrode being connected to the first reference electrode by a potentiometric circuit for providing a measure of the potential difference between the first and second reference electrodes, said potential difference between the first and second reference electrodes being used to calculate the second component of current and thereby provide a measure of the concentration of the analyte which has reduced or no dependency on the concentration of the interferent.
Typically, the interferent cohabits with the analyte gas or is generated by the counter electrode.
Preferably, the electrochemical cell is a gas sensor.
Preferably also, the analyte is carbon monoxide and the interferent is hydrogen.
Preferably, the first reference electrode is positioned to come into contact with minimised concentrations of interferent.
Preferably, the second reference electrode is at least partially in direct contact with the analyte/interferent without electrolyte between the second reference electrode and analyte/interferent.
The provision of a measure of the concentration of the analyte which has reduced or has no dependency on the concentration of the interferent may be achieved by scaling and subtracting the inverse logarithm of the potential difference between the first and second reference electrodes from the total current measured at the working electrode.
According to a third aspect of the present invention there is provided a method for designing an electrochemical cell (1) for sensing an analyte in the presence of an interferent, the electrochemical cell comprising a reference electrode (4) and a working electrode (2) connected by a potentiostatic circuit (7), the analyte reacting at the working electrode (2) giving a first component of current, the interferent undergoing an electrochemical reaction giving a second component of current, the presence at the reference electrode (4) of the interferent leading to a shift in the potential of the reference electrode, wherein the shift in the potential of the reference electrode (4) and the action of the potentiostatic circuit (7) leads to a third component of current, the method comprising the steps of:
selecting the properties of the electrochemical cell which affect the magnitude of the third component of current so that the third component of current acts to partially, completely or over compensate for the second component of current.
According to a fourth aspect of the present invention there is provided a method for calculating the concentration of an analyte in the presence of an interferent using an electrochemical cell comprising a working electrode and a first reference electrode connected by a potentiostatic circuit and also a second reference electrode, the interferent not affecting the reference electrode thus avoiding a shift in the potential of the reference electrode, the analyte reacting at the working electrode giving a first component of current, the interferent undergoing an electrochemical reaction giving a second component of current, the second reference electrode being configured so as to be exposed to the interferent, the interferent causing a shift in the potential of the second reference electrode, the second reference electrode being connected to the first reference electrode by a potentiometric circuit for providing a measure of the potential difference between the first and second reference electrodes, the method comprising the steps of:
measuring the potential difference between the first and second reference electrodes;
thereby calculating the second component of current; and
thereby providing a measure of the concentration of the analyte which has reduced or no dependency on the concentration of the interferent.
According to a fifth aspect of the present invention there is provided computer software comprising program code which, when loaded onto a computer, causes it to calculate the concentration of an analyte in the presence of an interferent by the method of the fourth aspect.